Membrane separation is used in various industries, for example, to remove contaminants from a gas mixture, or to recover components of value. Simple, one-step membrane processes are useful for achieving bulk separations, but are less useful when high levels of removal and recovery are desired. In such cases, multiple similar or dissimilar separation steps are often needed, and such combinations are familiar to the gas separation industry. For example, U.S. Pat. Nos. 5,089,033 and 5,199,962 describe combinations of condensation and membrane separation. U.S. Pat. Nos. 4,654,047 and 6,161,397 describe hybrid processes that use cryogenic and non-cryogenic separation steps. U.S. Pat. No. 4,772,295 describes processes using absorption in conjunction with membrane separation. U.S. Pat. Nos. 4,863,492 and 6,183,628 disclose methods for combining pressure swing adsorption (PSA) with membrane separation, and numerous patents, including U.S. Pat. Nos. 6,572,679 and 6,630,011, describe multi-step and multi-stage membrane separation processes. In the hybrid process, the membrane step or system may precede the non-membrane step or system, or vice versa.
In such processes, it is known to return the non-product stream from the second separation step to the inlet of the first step, to create a processing loop, thereby increasing product recovery. Some loss of gas pressure is inevitable as gas passes through the various unit operations, so the recycle gas has to be recompressed before it can re-enter the first separation step. This requires a recycle compressor to be included in the processing loop or, if there is a feed compressor upstream of the feed inlet to the first separation step, for the recycle gas to be passed again through the feed compressor. Either process consumes energy, thereby reducing the efficiency of the overall process. If a recycle compressor is needed, this adds to the capital costs and operating complexity of the process.
Thus, there remains a need for more energy-efficient and simple gas separation processes.